Many well completions involve setting a liner, casing, or other tubular string within a portion of a hole or wellbore. In some wells, such as extended reach wells drilled from platforms or "islands," a string must be set in a slant drilled (i.e., inclined angle) portion of a deviated hole. The inclined angle (from vertical) of these deviated hole portions frequently approaches 90 degrees, i.e., horizontal, and sometimes exceeds 90 degrees. Current state-of-the-art techniques allow extensive drilling of wellbores at almost any incline angle, but problems have been experienced in completing long, highly deviated wellbores, especially related to the setting of pre-perforated casing or liner strings.
A liner or casing string may be pre-perforated before being run and set in a rotary or pre-drilled wellbore in order to avoid a downhole perforating step. Although the drill string and drill bit used to cut the hole is typically rotated, thereby avoiding static friction drag forces which retard the pipe string from sliding into the hole, shape and other limitations of the tubulars being run typically precludes rotation. The tubulars being set are typically larger in diameter than the typical drill string and the torsional forces needed to rotate the casing or liner can be greater than the torsional strength of the pre-perforated tubulars or greater than the available rotary torque. Casing or liner strings are therefore normally run (i.e., slid) into the hole without drag reducing rotation.
Running tubulars in highly deviated holes can result in a significantly increased risk of a stuck tubular, especially when running a pre-perforated liner or casing. A pre-perforated casing or liner pipe string may become differentially stuck before reaching the desired setting depth during running into a deviated or high drag hole, especially when the perforations create added drag and if the incline angle exceeds a critical angle. The critical angle is defined as when the weight of the casing or liner in the wellbore produces more drag force than the component of weight tending to slide the casing or liner down the hole. If sufficient additional force (up or down) cannot be applied to a stuck tubular, the result may be a stuck tubular and effective loss of the well. Even if a stuck string is avoided, the forces needed to unstick the tubular may cause serious damage to the drill pipe or tubulars, especially when the tubular is pre-perforated.